The geochemistry of mafic and ultramafic from the Archaean greenstone belts of Sierra Leone

AbstractThe Archaean (c. 2800 Ma) ultramafic rocks in eastern Sierra Leone cut basalt lavas and are mostly olivine-rich cumulates either iron-rich (Fo85–86) and derived from a basaltic or picritic parent, or more magnesian (Fo92–93) derived from an ultramafic melt with c. 18–25 wt. % MgO. In central Sierra Leone the ultramafic rocks are lavas predating tholeiitic basalts.The basalts show a wide variation in Zr/Y, suggesting that garnet was present in the source region of some of these rocks but not others. This implies that melting took place at different depths in the mantle. The REE evidence for basaltic rocks in the upper part of the Nimini belt succession suggests that they were derived from a mantle source region which had already suffered melt extraction. Ti/Zr ratios in the basaltic rocks are also variable and individual belts define different trends on a Ti vs. Zr plot implying that the basaltic rocks evolved in geographically distinct magma chambers. It is likely that the basaltic rocks evolved from a parental liquid with Ti/Zr = 90 via shallow level crystal fractionation. The source region for these rocks therefore had a lower than chondritic Ti/Zr.There are two possible models for the basaltic and ultramafic magmas in the Sierra Leone greenstone belts. First that the ultramafic and basaltic liquids were derived from mantle diapirs of differing size, but originating in the same region of the mantle. Ultramafic liquids were produced in small diapirs, which store large melt fractions, and basaltic liquids in larger diapirs which segregate larger melt fractions. A second model is based upon the double diffusion process suggested for magma chambers at mid-ocean ridges and involves a transient magma chamber from which basalts, derived from parental ultramafic liquids, are erupted, with ultramafic liquids rising directly to the surface when the magma chamber is frozen. The available data does not discriminate between these two models.

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The Ferguson Lake deposit: an example of Ni–Cu–Co–PGE mineralization emplaced in a back-arc basin setting?

Canadian Journal of Earth Sciences ◽

10.1139/cjes-2017-0185 ◽

2018 ◽

Vol 55 (8) ◽

pp. 958-979 ◽

Cited By ~ 1

Author(s):

P. Acosta-Góngora ◽

S.J. Pehrsson ◽

H. Sandeman ◽

E. Martel ◽

T. Peterson

Keyword(s):

Mineral Exploration ◽

Ultramafic Rocks ◽

Geodynamic Setting ◽

Ocean Ridges ◽

Tectonic Environment ◽

Pge Mineralization ◽

Extensional Tectonic ◽

Basaltic Composition ◽

Greenstone Belts ◽

Back Arc

The world’s largest Ni–Cu–Platinum group element (PGE) deposits are dominantly hosted by ultramafic rocks within continental extensional settings (e.g., Raglan, Voisey’s Bay), resulting in a focus on exploration in similar geodynamic settings. Consequently, the economic potential of other extensional tectonic environments, such as ocean ridges and back-arc basins, may be underestimated. In the northeastern portion of the ca. 2.7 Ga Yathkyed greenstone belt of the Chesterfield block (western Churchill Province, Canada), the Ni–Cu–Co–PGE Ferguson Lake deposit is hosted by >2.6 Ga hornblenditic to gabbroic rocks of the Ferguson Lake Igneous Complex (FLIC), which is metamorphosed up to amphibolitic facies. The FLIC has a basaltic composition (Mg# = 31–72), flat to slightly negatively sloped normalized trace element patterns (La/YbPM = 0.7–3.5), and negative Zr, Ti, and Nb anomalies. The FLIC rocks are geochemically similar to the 2.7 Ga back-arc basin tholeiitic basalts from the adjacent Yathkyed and MacQuoid greenstone belts (Mg# = 30–67; La/YbPM = 0.3–3.0), but the Ferguson Lake intrusions appear to be more crustally contaminated. We interpret the FLIC to have formed in an equivalent back-arc basin setting. This geodynamic setting is rare for the formation of Ni–Cu–PGE occurrences, and only few examples of this tectonic environment (or variations of it, e.g., rifted back-arc) are found in other Proterozoic and Archean sequences (e.g., Lorraine deposit, Quebec). We suggest that back-arc basin-derived mafic rocks within the Yathkyed and other Neoarchean greenstone belts of the Chesterfield block (MacQuoid and Angikuni) could represent important targets for future mineral exploration.

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The Rustenburg Layered Suite formed as a stack of mush with transient magma chambers

Nature Communications ◽

10.1038/s41467-020-20778-w ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Zhuosen Yao ◽

James E. Mungall ◽

M. Christopher Jenkins

Keyword(s):

Magma Chamber ◽

Fractional Crystallization ◽

Bushveld Complex ◽

Mineral Deposits ◽

Ultramafic Rocks ◽

Layered Intrusions ◽

Crustal Assimilation ◽

Magma Chambers ◽

Magma Chamber Processes ◽

Lower Crustal

AbstractThe Rustenburg Layered Suite of the Bushveld Complex of South Africa is a vast layered accumulation of mafic and ultramafic rocks. It has long been regarded as a textbook result of fractional crystallization from a melt-dominated magma chamber. Here, we show that most units of the Rustenburg Layered Suite can be derived with thermodynamic models of crustal assimilation by komatiitic magma to form magmatic mushes without requiring the existence of a magma chamber. Ultramafic and mafic cumulate layers below the Upper and Upper Main Zone represent multiple crystal slurries produced by assimilation-batch crystallization in the upper and middle crust, whereas the chilled marginal rocks represent complementary supernatant liquids. Only the uppermost third formed via lower-crustal assimilation–fractional crystallization and evolved by fractional crystallization within a melt-rich pocket. Layered intrusions need not form in open magma chambers. Mineral deposits hitherto attributed to magma chamber processes might form in smaller intrusions of any geometric form, from mushy systems entirely lacking melt-dominated magma chambers.

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Formation of the Rustenburg Layered Suite by assimilation – batch crystallization (ABC) and – fractional crystallization (AFC)

10.21203/rs.3.rs-36556/v1 ◽

2020 ◽

Author(s):

Zhuosen Yao ◽

James Mungall ◽

M Jenkins

Keyword(s):

Magma Chamber ◽

Fractional Crystallization ◽

Bushveld Complex ◽

Mineral Deposits ◽

Ultramafic Rocks ◽

Crystal Mush ◽

Magma Chambers ◽

Batch Crystallization ◽

Magma Chamber Processes ◽

Second Stage

Abstract The Rustenburg Layered Suite (RLS) of the Bushveld Complex of South Africa is a vast layered accumulation of mafic and ultramafic rocks. The layers are widely assumed to result from fractional crystallization from a melt-dominated magma chamber. We derive compositions of all units of the RLS with thermodynamic models of assimilation of crust by a komatiitic parent magma. Ultramafic U-type cumulate layers represent crystal mush produced by single stages of assimilation-batch crystallization (ABC). Anorthositic (A-type) magma mushes emerged by a second stage of batch crystallization during ascent of melts supernatant to mid-crustal U-type cumulates. Only the ferrobasaltic magma of the Upper and Upper Main Zone formed via classical assimilation-fractional crystallization (AFC) and ponded in a melt-dominated magma chamber that subsequently evolved by fractional crystallization. Mineral deposits associated with reversals between mafic and ultramafic layers, hitherto attributed to magma chamber processes, might form in small intrusions entirely lacking melt-dominated magma chambers.

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The Hf-Nd dichotomy: constraints from felsic, mafic and ultramafic rocks in the western Dharwar Craton, India

10.5194/egusphere-egu2020-21119 ◽

2020 ◽

Author(s):

Arathy Ravindran ◽

Klaus Mezger ◽

Srinivasan Balakrishnan

Keyword(s):

Dharwar Craton ◽

Ultramafic Rocks ◽

Earth Planet ◽

Nd Isotope ◽

Basaltic Rocks ◽

Mantle Evolution ◽

Depleted Mantle ◽

Greenstone Belts ◽

Ree Patterns ◽

Different Levels

The Hf-Nd dichotomy: constraints from felsic, mafic and ultramafic rocks in the western Dharwar Craton, India Arathy Ravindran1*, Klaus Mezger1, S. Balakrishnan21Institut f&#252;r Geologie, Universit&#228;t Bern, Bern, Switzerland2Department of Earth Sciences, Pondicherry University, Puducherry, India(*correspondence: [email protected])The small extend of exposed Hadean-Paleoarchaean (>3.2 Ga) rocks in the global record poses a major challenge in interpreting Earth&#8217;s early crust-mantle evolution. This results in major uncertainty in the degree and extent of heterogeneity of the Archaean mantle (e.g. Nebel et al., 2014). Isotope systems like 176Lu-176Hf and 147Sm-143Nd are powerful tools in tracing the degree of mantle depletion and the influence of concomitant continental crust formation. However, these isotope systems are apparently decoupled in Archaean ultramafic rocks (e.g. Hoffmann and Wilson, 2017). Hence, the Hf-Nd isotope dichotomy in ultramafic rocks requires a detailed study of cratonic areas hosting granitoids spatially associated with greenstone belts and ultramafic rocks, as it is the case in the western Dharwar Craton (~3.4 Ga) of India.The 3.25 Ga old rhyolitic to basaltic rocks of the craton that have flat, mantle-like REE patterns also have 147Sm-143Nd and 176Lu-176Hf signatures &#8216;coupled&#8217; along a trend &#603;176Hf = 1.55 * &#603;143Nd + 1.21 (Vervoort et al., 2011). The minor depletion recorded in these rocks is a result of mixing at different levels between a 3.6 Ga old mafic crust (Ravindran et al., 2020) and the contemporary depleted mantle. The tonalite-trodhjemite-granodiorite (TTG) gneisses have similar isotope ratios and their petrogenesis involved the mafic crust until 3.3 Ga, after which reworked crust was the major component. Komatiitic rocks (MgO=15-30%; Na2O+K2O <1%; (Gd/Yb)N=0.6-1.8) with an age of 3.35 Ga have high and variable initial &#603;Hf (+3 to +20) compared to their initial &#603;Nd (+1.0 to +3.5). These ultramafic rocks have decoupled Hf-Nd signatures which is uncommon for the mafic and felsic rocks in the craton. This further shows that the mantle composition was more heterogeneous in the early Archaean than today. It is also possible that the presence of garnet in the mantle source was an important parameter which influenced the composition of the early Archaean crust.&#160;&#160;References:Hoffmann, J. E., Wilson, A. H., 2017. Chem. Geo. 455, 6-21Nebel, O., Campbell, I. H., Sossi, P. A., Van Kranendonk, M. J., 2014. Earth. Planet. Sci. Lett. 397, 111-120Ravindran, A., Mezger, K., Balakrishnan, S., Kooijman, E., Schmitt, M., Berndt, J., 2020. Prec. Res. 337Vervoort, J., Plank, T., Prytulak, J., 2011. Geochim. Cosmochim. Acta 75, 5903-5926

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Peculiarities of the distribution of rare-earth elements in the ores of some gold deposits of Eastern Transbaikalia

Proceedings of higher educational establishments Geology and Exploration ◽

10.32454/0016-7762-2018-5-48-58 ◽

2018 ◽

pp. 48-58

Author(s):

B. N. Abramov

Keyword(s):

Rare Earth ◽

Rare Earth Elements ◽

Gold Deposits ◽

Volatile Components ◽

Magma Chambers ◽

Eastern Transbaikalia ◽

Different Depths ◽

Degree Of Differentiation ◽

Gold Sulfide ◽

Ree Patterns

The distribution of rare-earth elements (REE) in ores of gold deposits of East Transbaikalia has shown that the ore-bearing magma chambers have different depths and degrees of differentiation. The greatest degree of differentiation was within the magmatic foci (Eu/Eu* — 0,29—0,32; Rb/Sr — 0,98—1,40), which are the sources of gold-quartz-arsenopyrite ores, the magmatic sources of the gold-quartz and gold-sulfide-quartz ores (Eu/Eu* — 0,53—0,72; Rb/Sr of 0,10 to 0,54) had lesser degree of differentiation. Magma chambers that are sources for the gold-quartz-arsenopyrite ores (Eu/Sm — 0,08—0,14), were at shallower depths than those for gold-quartz and gold-sulfide-quartz ores (Eu/Sm — 0,11—0,19). The formation of gold-quartz-arsenopyrite ores took place at the magma chambers, largely enriched in volatile components, it is indicated by the existence of a significant tetrad effects in REE patterns of (T1-4 - 0,80; 1,15; 1,16).

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DISCOVERY OF NEW KIMBERLITE FIELDS ON THE SOUTHERN SLOPE OF THE LEONE-LIBERIAN SHIELD (Sierra Leone)

Ukrainian Geologist ◽

10.53087/ug.2021.1-2(44-45).238960 ◽

2021 ◽

pp. 118-126

Author(s):

Oleksandr BOBROV ◽

Sergii KLOCHKOV ◽

Serhiy KAKARANZA ◽

Oleksandr KAKARANZA ◽

Yurii FEDORISHIN ◽

...

Keyword(s):

Sierra Leone ◽

Ultramafic Rocks ◽

Southern Slope ◽

Rock Matrix ◽

Geophysical Research ◽

Optimal Sequence ◽

Space Images ◽

Complex Morphology ◽

Geomorphological Analysis ◽

Contact Part

During 2017–2018 not far from Sewafeh town, Kono province (Republic of Sierra Leone), we identified a number of previously unknown manifestations of kimberlite magmatism in the form of a system of individual veins localized at the contact of the migmatite basement and Archean ultrabasic massifs, or in the immediate vicinity of ultramafic massifs, which is part of the rocks of the greenstone belt. The optimal sequence of conducting remote sensing studies, such as interpretation of space images of various resolution, neotectonic and geomorphological analysis, SRTM modeling, and then field geological and geophysical research have facilitated this discovery. According to drilling data, kimberlites in different spots of their occurrence (Punduru 1 area) are represented by subvolcanic phlogopite-olivine (with perovskite), and olivine varieties, as well as veins of numerous intensely metasomatic altered kimberlite breccias (Yomby area). Veins of subvolcanic kimberlites are concentrated in the contact part with ultramafic massifs of magmatic and lava (metakomatiite xenoliths) genesis. Kimberlites are the youngest vein formations in the area, crossing even vein pegmatites, the generation of which was provoked by the intrusion of ultramafic rocks in the basement migmatites (Cederholm effect). Kimberlites are present in the section of wells in the form of separate veins of complex morphology and thickness from a few centimeters to 45 cm. In well P1-2 at a depth of 92 m, these are represented by micro porphyry kimberlites of the basaltoid type with microlithic groundmass, altered by secondary metasomatic processes. Porphyry inclusions are represented by pseudomorphs of carbonate-serpentine composition after olivine and rare phlogopite flakes. Olivine crystals of the second-generation act as micro porphyry inclusions. The rock matrix is carbonate. Carbonate is represented by finely crystalline calcite, or replaced by dolomite. In addition to olivine, the groundmass contains relics or pseudomorphs after phlogopite, as well as magnetite, perovskite (it can be replaced by magnetite), secondary apatite. The kimberlites of the Bambawo area are represented by sub-volcanic porphyry basaltoid kimberlites, autolithic kimberlites and kimberlite xenotuff breccias.

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Oxygen-Hafnium-Neodymium Isotope Constraints on the Origin of the Talnakh Ultramafic-Mafic Intrusion (Norilsk Province, Russia)

Economic Geology ◽

10.5382/econgeo.4743 ◽

2020 ◽

Vol 115 (6) ◽

pp. 1195-1212 ◽

Cited By ~ 1

Author(s):

Kreshimir N. Malitch ◽

Elena A. Belousova ◽

William L. Griffin ◽

Laure Martin ◽

Inna Yu. Badanina ◽

...

Keyword(s):

Crustal Contamination ◽

Ultramafic Rocks ◽

Magma Chambers ◽

Nd Isotope ◽

Crustal Component ◽

Depleted Mantle ◽

Magma Sources ◽

A Minor ◽

T Values

Abstract The ultramafic-mafic Talnakh intrusion in the Norilsk province (Russia) hosts one of the world’s major platinum group element (PGE)-Cu-Ni sulfide deposits. This study employed a multitechnique approach, including in situ Hf-O isotope analyses of zircon combined with whole-rock Nd isotope data, in order to gain new insights into genesis of the Talnakh economic intrusion. Zircons from gabbrodiorite, gabbroic rocks of the layered series, and ultramafic rocks have similar mantle-like mean δ18O values (5.39 ± 0.49‰, n = 27; 5.64 ± 0.48‰, n = 34; and 5.28 ± 0.34‰, n = 7, respectively), consistent with a mantle-derived origin for the primary magma(s) parental to the Talnakh intrusion. In contrast, a sulfide-bearing taxitic-textured troctolite from the basal part of intrusion has high δ18O (mean of 6.50‰, n = 3), indicating the possible involvement of a crustal component during the formation of sulfide-bearing taxitic-textured rocks. The Hf isotope compositions of zircon from different rocks of the Talnakh intrusion show significant variations, with ɛHf(t) values ranging from –3.2 to 9.8 for gabbrodiorite, from –4.3 to 11.6 for unmineralized layered-sequence gabbroic rocks, from 2.3 to 12 for mineralized ultramafic rocks, and from –3.5 to 8.8 for mineralized taxitic-textured rocks at the base of the intrusion. The significant range in the initial 176Hf/177Hf values is ascribed to interaction of distinct magma sources during formation of the Talnakh intrusion. These include (1) a juvenile source equivalent to the depleted mantle, (2) a subcontinental lithospheric source, and (3) a minor crustal component. Initial whole-rock Nd isotope compositions of the mineralized taxitic-textured rocks from the base of the intrusion (mean ɛNd(t) = –1.5 ± 1.8) differ from the other rocks, which have relatively restricted ranges in initial ɛNd (mean ɛNd = 0.9 ± 0.2). The major set of ɛNd values around 1.0 at Talnakh is attributed to limited crustal contamination, presumably in deep magma chambers, whereas the smaller set of negative ɛNd values in taxitic-textured rocks is consistent with greater involvement of a crustal component and reflects an interaction with the wall rocks during emplacement.

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Exploring Mechanisms and Conditions for the Generation of Self-propagating Dykes

10.5194/egusphere-egu2020-12479 ◽

2020 ◽

Author(s):

Herbert Wallner ◽

Harro Schmeling

Keyword(s):

Yield Stress ◽

Shear Failure ◽

Source Region ◽

Shear Bands ◽

Shear Stresses ◽

Crustal Rock ◽

Stress Concentrations ◽

Magma Chambers ◽

Two Phase ◽

Matrix Extension

Within the scope of our project &#8220;Modelling melt ascent through the asthenosphere-lithosphere-continental crust system: Linking melt-matrix-two-phase flow with dyke propagation&#8221; it is necessary to implement mechanisms with appropriate conditions to generate dykes which are propagating independently.Conditions for self-propagating depend on the density contrast of melt and rock and the geometry of the fracture. Certain limits for the fluid-filled volume and dyke width must be reached. The height must be longer than the Bouguer length. To satisfy these conditions enough melt under overpressure must be available in the source region to supply the growing dyke.A known and accepted mechanism for dyke generation is a tension fracture whichs opening space immediately is filled by fluid melt. The normal stress due to expansion of the magma on the wallrock causes tension therein parallel to the melt front. In brittle material the yield stress for extension is very low and the confining cold rock easily cracks.With depth pressure, temperature and ductility of crustal rock and consequently the yield stress for the tensile cracking increases. Furthermore, the background permeability or connectivity, and finally the height of fluid columns decrease and the fluid overpressure is not high enough to exert matrix extension. Another dyke initiation mechnism must be found for the deeper parts of the crust.A not smooth melt front - and pillows are often seen on top of magma chambers &#8211; provides shear stresses and stress concentrations. Above a certain yield stress for shear failure shear bands start to evolve. In such a network of fracture zones permeability should increase. Melt may intrude, coalesce bands and develop a growing dyke. Such a local scenario will be modelled and results presented. A further aim is the parametrisation of these mechanisms.

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Magma chamber formation by magma intrusion into the Earth's crust

10.5194/egusphere-egu2020-9566 ◽

2020 ◽

Author(s):

Ivan Utkin ◽

Oleg Melnik

Keyword(s):

Magma Chamber ◽

Flow Rate ◽

Structural Features ◽

Numerical Dissipation ◽

Magma Chambers ◽

Magma Intrusion ◽

Magma Flow ◽

Formation Of Cracks ◽

Heating Up ◽

Magma Flow Rate

The main mechanism of transport of magma in the Earth&#8217;s crust is the formation of cracks, or dikes, through which the melt moves towards the surface under the action of buoyancy forces and tectonic stresses. Due to the structural features of the crust or external stress fields, dikes often do not reach the surface, but penetrate the localized region in which the rocks melt, leading to the formation of magmatic chambers, whose volume can exceed thousands of cubic kilometers. We present a model of the formation of a magma chamber during the intrusion of dikes at a given flow rate. The model is based on the solution of heat equation and considers the actual melting diagrams of magma and rocks. It Is shown that, in case of magmatic fluxes typical of island arc volcanoes, magma chambers are formed over hundreds of years from the beginning of magma intrusion. The influence of the magma flow rate, the size of the dikes and their orientation on the volume of the formed magma chamber and its shape was investigated. The size of the chamber significantly exceeds the area of dike intrusion due to the displacement of magma and rocks of the crust, their heating up and melting. To calculate displacement of rock and magma in a numerical simulation, a hybrid method based on PIC/FLIP interpolation is developed, making it possible to avoid unphysical mixing due to numerical dissipation, thus preserving the fine details of the formed magma chamber.This work was supported by RFBR, project number 18-01-00352

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Early basic magmatism in the evolution of Archaean high-grade gneiss terrains: an example from the Lewisian of NW Scotland

Mineralogical Magazine ◽

10.1180/minmag.1987.051.361.02 ◽

1987 ◽

Vol 51 (361) ◽

pp. 345-355 ◽

Cited By ~ 9

Author(s):

H. R. Rollinson

Keyword(s):

Trace Element ◽

Source Region ◽

High Grade ◽

Ocean Ridges ◽

Layered Gabbro ◽

Basic Magmatism ◽

Melting Regime ◽

High Grade Metamorphism ◽

Tonalitic Gneiss ◽

Common Parent

AbstractAmphibolite blocks from an Archaean (2.9 Ga) trondhjemite-agmatite complex in the Lewisian at Gruinard Bay have a varied trace element and REE content. Whilst some of the variability is attributable to element mobility during high-grade metamorphism and subsequent trondhjemite magmatism, it is for the main part considered to be a primary feature of the amphibolites. The observed trace element and REE chemistry is best explained in terms of source region heterogeneity and suggests a melting regime comparable with that beneath certain types of mid-ocean ridge. There are geochemical similarities between the amphibolites and the Lewisian layered gabbro-ultramafic complexes, and the two may represent the derivative liquid and associated cumulates respectively from a common parent magma. Thus there is a parallel between the processes which generated some Archaean amphibolites and layered gabbro complexes and those operating beneath modern ocean ridges. Hornblendite and amphibolite pods enclosed within tonalitic gneiss, also found as blocks in the agmatite complex, are geochemically distinct from the main group of amphibolites and are probably of calc-alkaline parentage.

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